Night vision devices are used extensively by the military to enable operations at night under low ambient light conditions. Applications include goggles worn by foot soldiers, as well as apparatus used by soldiers in military vehicles and aircraft. Applications further include imaging apparatus included in unmanned ground and airborne vehicles. Similar night vision equipment is also used by law enforcement, and for certain other civilian applications.
While the obvious application of a night vision device is for use under low light conditions, there are often circumstances where it is desirable to use a night vision device under a wide range of light conditions ranging from moonless starlight to full daylight. For example, a scene viewed at night may be illuminated artificially in some areas and not in others areas, causing the light level to vary widely as a soldier looks and/or travels across the scene between brightly lit areas and areas that are not illuminated. If the soldier removes the night vision device when looking at illuminated areas, it may take several seconds for his or her eyes to adjust, and then several more seconds may be required for the soldier's eyes to readjust back to use of the night vision device. A night vision device may also be sensitive to wavelengths that are not visible to the human eye, making it desirable to use the device under all lighting conditions.
Traditional night vision devices use an analog “image intensifier” (“II”) tube to enhance an image obtained at low ambient light levels. Intensification factors can range from 1000× (moonlight) up to 20,000× (moonless, clear starlight) and even up to 50,000× (deeply overcast starlight) and higher.
However, II tubes are expensive, complicated, and low in manufacturing yield of components and tube assemblies. II tubes can also be heavy and bulky, compare to digital focal plane arrays, can be vulnerable to direct sunlight and laser radiation, and must be replaced periodically.
II tubes also typically have limited resolution when used under daylight conditions, and they are difficult to optically fuse with other sensors that operate at wavelengths outside of the visible band.
Digital low light focal plane arrays are known for long wave and mid wave infrared detection. However, attempts in the prior art to create digital low light focal plane arrays for visible light have failed, due at least in part to electron gain limitations associated with conventional silicon photodiodes. Some attempts have been made to create low visible light digital focal plane arrays using intensified charge-coupled device CMOS (ICCD/CMOS) imagers in lieu of II tubes. However, such devices can be heavy, bulky, and overly expensive, and can require a significant amount of power from batteries or from a power supply, which must be carried by a foot soldier.
Attempts have also been made to create low visible light digital focal plane arrays using low light level (LLL) CCD/CMOS imagers. An example at low light is shown in FIG. 1A, and an enlargement of a portion of FIG. 1A is presented in FIG. 1B. However, current LLL CCD/CMOS imagers have proven to be incapable of night-time performance below ¼ moon conditions, due to high dark current and little to no electron gain.
What is needed, therefore, is a solid state digital imaging sensor for night vision devices that is not bulky or overly expensive, has low power requirements, provides high performance images at low light levels down to overcast starlight and high contrast unsaturated images in full daylight, and can be digitally fused with devices that are sensitive to wavelengths outside of the visible band.